# How do traffic sensors work?

**URL:** <https://boards.straightdope.com/t/how-do-traffic-sensors-work/34156>\
**Category:** Factual Questions\
**Created:** [September 22, 2000, 4:41pm UTC](https://boards.straightdope.com/t/how-do-traffic-sensors-work/34156 "2000-09-22T16:41:54Z")\
**Posts on this page:** 4\
**Page:** 2

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**Author:** ![Silo](https://avatars.discourse-cdn.com/v4/letter/s/ac91a4/32.png) [@Silo](https://boards.straightdope.com/u/Silo)\
**Post date:** [September 25, 2000, 7:33am UTC](https://boards.straightdope.com/t/how-do-traffic-sensors-work/34156/21 "2000-09-25T07:33:11Z")

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Wow, Chronos, I just noticed you’re a moderator. Congrads.

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**Author:** ![yabob](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/yabob/32/2821_2.png) [@yabob](https://boards.straightdope.com/u/yabob)\
**Post date:** [September 25, 2000, 4:14pm UTC](https://boards.straightdope.com/t/how-do-traffic-sensors-work/34156/22 "2000-09-25T16:14:24Z")

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For the record, ricepad, I remember the pressure plates. Occasionally, if someone stopped too far back from the light, they would be sitting on top of the plate and the light wouldn’t change.

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**Author:** ![SirRay](https://avatars.discourse-cdn.com/v4/letter/s/cc9497/32.png) [@SirRay](https://boards.straightdope.com/u/SirRay)\
**Post date:** [September 25, 2000, 5:23pm UTC](https://boards.straightdope.com/t/how-do-traffic-sensors-work/34156/23 "2000-09-25T17:23:46Z")

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Totally off topic (well, perhaps vaguely related), but I think I asked this awhile ago and didn’t get any answers (maybe for good reason) .  
Here on beautiful Long Island, there are a number of left turn signals triggered via sensors (BTW almost everyone I’ve seen in NY and Jersey has the tell-tale induction loop cut-out in the roadway, usually starting right behind the Stopline). Well, there is a sizable sub-set of, er, ‘Drivers’, who due to single digit IQs park their vehicles well behind the stop-line and wait for the light to change, which of course it never will because their vehicle is too far back to trigger the sensor. Much horn-blowing on my part doesn’t seem to bring any dawning of realization on their part, so I generally have to veer around them (rather dangerous) and pull in front to trigger the light myself. This, alas, happens all too frequently.  
Any suggestions (realistic ones, apart from violence) on what can be done to wake these brain-dead folks up?

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**Author:** ![occ](https://avatars.discourse-cdn.com/v4/letter/o/8c91f0/32.png) [@occ](https://boards.straightdope.com/u/occ)\
**Post date:** [September 25, 2000, 7:39pm UTC](https://boards.straightdope.com/t/how-do-traffic-sensors-work/34156/24 "2000-09-25T19:39:34Z")

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> [@](#):
>
> \*Originally posted by Giraffe \*  
> \*\*If it’s an inductance loop, does anyone know the threshold? (I know my bicycle can’t cause the light to change. Can a motorcycle? What about newer economy cars? Is there enough metal in a GEO to change a traffic light?)
> 
> Just curious… \*\*

When I’m on my bike, it generally doesn’t set off the sensors. Wonder how that would hold up in court if a cop saw you running a light because it wouldnt change?

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